Method and apparatus for welding with CV control

ABSTRACT

A method and apparatus for welding is disclosed. The output is preferably a cyclical CV MIG output, and each cycle is divided into segments. An output parameter is sampled a plurality of times within one or more of the segments. The CV output is controlled within the at least one segment in response to the sampling. The parameter is output power, a resistance of the load, an output current, an output voltage, or functions thereof in various embodiments. The control loop is preferably a PI or PID loop. The loop may be applied only within a window. The set point may be taught or fixed. The system can be used to weld with a controlled arc length.

RELATED APPLICATIONS

This is a continuation of, and claims the benefit of the filing date of,U.S. patent application Ser. No. 11/006,280, filed on Dec. 7, 2004,which is a continuation of, and claims the benefit of the filing dateof, U.S. patent application Ser. No. 10/268,514, filed on Oct. 9, 2002,entitles method and Apparatus For Welding With CV Control, which issuedon Jun. 21, 2005 as U.S. Pat. No. 6,909,067.

FIELD OF THE INVENTION

The present invention relates generally to the art of welding powersupplies. More specifically, it relates to welding systems that have aCV control.

BACKGROUND OF THE INVENTION

There are many known types of welding systems used for many differentwelding processes. Welding system, or system for welding, as usedherein, includes one or more of a power source, controller, wire feeder,and a source of gas, it may also include peripherals such as robots etc.Welding power supply, as used herein, includes any device capable ofsupplying welding, plasma cutting, and/or induction heating powerincluding power circuitry, control circuitry and other ancillarycircuitry associated therewith.

One type of welding system includes a CC welding power source. A CCwelding power source, as used herein, is a current controlled powersource, wherein the output current is monitored, and the output isadjusted to provide the desired current. The current may be constantduring the welding process, or may have a desired waveform of varyingcurrent.

Another type of welding system includes a CV power source. A CV weldingpower source, as used herein, is a voltage controlled power source,wherein the output voltage is monitored, and the output is adjusted toprovide the desired voltage. The voltage may be constant during thewelding process, or may have a desired waveform of varying voltage. SomeCV welding power sources use a current command, and adjust the currentin response to the monitored voltage, thereby adjusting the arc voltageby the change in current across the load.

One known welding process is a pulsed MIG process. The pulsed MIGprocess typically has a cyclical output having at least a peak segmentwith a relatively high current and a background segment with arelatively low current. Generally, it is desirable to maintain aconstant arc length during each segment, to help keep the processconsistent. Unfortunately, during the process the torch angle and/ordistance to the work piece may be inadvertently varied by the welder.This results in undesirable changes to the arc length.

Prior art control schemes attempted to control arc length by using a CVcontrol because arc (load) voltage is generally related to arc lengthfor a given welding current. Thus, keeping a constant voltage for agiven current maintains a constant arc length. Most prior art CVcontrollers for pulsed MIG average the arc voltage over at least onecycle, and then adjust the output for the next cycle in response to theaverage. The average has been taken over a plurality of cycles, anentire cycle, over only the background segment, or over only the peaksegment. Because the average is taken over at least one cycle, theoutput (and arc length) is not changed until at least the followingpulse cycle.

Averaging over one or more cycles yields a control scheme with arelatively slow response. The slow response forced a choice between atighter arc (which provides better control of the weld puddle), andexplosive clearing of shorts and spatter (which adversely affects thequality of the weld).

Accordingly, a control scheme for pulsed MIG that provides fast responseto control the arc, and does not result in undesirable short clearing,is needed. Preferably such a control scheme will be able to be used inother processes such as CMT (controlled metal transfer or short circuit)or AC MIG.

SUMMARY OF THE PRESENT INVENTION

According to a first aspect of the invention a method of controlling awelding power supply includes providing a cyclical CV output anddividing each cycle into at least two segments. An output parameter issampled a plurality of times within at least a one of the segments. TheCV output is controlled within the at least one segment in response tothe sampling.

According to a second aspect of the invention a system for providingwelding power includes a welding power supply, a feedback module, and acontroller. The power supply has a CV output with an output voltage, andis a series of cycles, each with at least two segments. The feedbackmodule is connected to the output, and provides feedback to thecontroller. The controller has a sampling module that receives thefeedback, and an output control signal responsive to the samplingmodule.

The parameter is sampled a plurality of times within at least a secondof the segments, or each of the segments, and the CV output iscontrolled within the second segment, or each of the segments, inresponse thereto, in alternative embodiments.

Sampling the parameter includes sampling an output power, a resistanceof the load, an output current, an output voltage, or functions thereofin various embodiments.

The sampled voltage is compared to a desired voltage to obtain an error,and the output is controlled in response to the error in anotherembodiment. A gain, preferably dependent upon the segment in which thesampling is taken, is applied to the error, and/or a PI or PID controlloop is applied in various embodiments.

The desired voltage is taught at a plurality of data points, or is froma fixed set of data points in other embodiments

The output is controlled by changing an output current, an outputvoltage, or an output power in some alternatives. A minimum outputcurrent is provided in another alternative.

The sampling is performed in a window within the segment, and the windowdoes not extend to the beginning or the end of each segment in oneembodiment. The output may be controlled to be at least partially a CCoutput outside of the window.

The arc length is controlled to be a desired length in one embodiment.

The output is a pulsed MIG output, CMT output, and AC MIG output invarious embodiments.

The controller includes an error module connected to the samplingmodule, has a desired voltage input in another embodiment. Thecontroller has a gain module connected to the error module, and/or a PIor PID control modules in additional embodiments. A desired voltageteaching module, or a fixed data point set, provide the desired voltageinput in other embodiments.

The controller includes a window module connected to the samplingmodule, and the controlling in response to the sampling is performed ina window within each segment in another embodiment. The systemcontroller further includes a CC control module, and the output controlmodule responsive to one or both of the CC control module and thesampling module, depending on the state of the window module.

The controller is an arc length controller in another embodiment.

Other principal features and advantages of the invention will becomeapparent to those skilled in the art upon review of the followingdrawings, the detailed description and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is block diagram of a system for providing welding power inaccordance with the present invention;

FIG. 2 is a graph showing an output waveform in accordance with thepresent invention;

FIG. 3 is a block diagram of a controller in accordance with the presentinvention; and

FIG. 4 illustrates a welding arc.

Before explaining at least one embodiment of the invention in detail itis to be understood that the invention is not limited in its applicationto the details of construction and the arrangement of the components setforth in the following description or illustrated in the drawings. Theinvention is capable of other embodiments or of being practiced orcarried out in various ways. Also, it is to be understood that thephraseology and terminology employed herein is for the purpose ofdescription and should not be regarded as limiting. Like referencenumerals are used to indicate like components.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

While the present invention will be illustrated with reference to aparticular system for providing welding power, with particularcomponents, and used for particular processes, it should be understoodat the outset that the invention may also be implemented with otherwelding systems, processes, components, controllers, etc.

Generally, the invention provides for controlling a welding process,such as pulse CV MIG welding, by sampling the output (or arc) voltagewithin each segment of the output waveform, and adjusting the output inresponse to the sampling and a desired voltage, for a very fast control.The output is adjusted in response to each sample, or in response to aplurality of samples, but the adjustment is made within the cycle inwhich the sample or samples are taken.

Preferably, the control scheme provides each segment with a gain, andprovides a number of taught or preset data points (of desired voltage)within each segment. The integral term is taken over the current and/orpast cycles in various embodiments. The controller provides a controlloop with an error, and either a PI or PID control loop in the preferredembodiment. The control is preferably adaptative in that it is taught atvarious points and it changes the output within a cycle based on theoutput of that cycle.

The invention may also be implemented with other processes, particularlymulti-segment processes like CMT and AC MIG that benefit from a stablearc length.

Because the control is based on output voltage feedback, which isrelated to arc length, the present invention is readily used to performa process with constant arc length.

Referring now to FIG. 1, a system for providing welding power 100 inaccordance with the present invention includes a power source 102 and awire feeder 106, that are controlled by a controller 104 to produce awelding arc 108. The various components may be disposed in separate orcommon housings, and may be in more than one housing (a portion ofcontroller 104 can share a housing with power source 102, while anotherportion share a housing with wire feeder 106).

Controller, as used herein, includes digital and analog circuitry,discrete or integrated circuitry, DSPs, microprocessors, etc., andsoftware, hardware and firmware, located on one or more boards, used tocontrol a device such as a power supply. The preferred embodimentsprovides that power source 102 is a Miller networked power source suchas that described in U.S. Pat. No. 6,329,636 and wire feeder 106 is aMiller networked wire feeder. Controller 104 is part of the commerciallyavailable components in this embodiment.

Preferably, system for providing welding power 100 is a CV MIG powersupply and provides a CV MIG output. Power source 102 of the preferredembodiment has a current command and provides an output current,preferably a pulsed output (and is thus a pulsed power supply) inresponse thereto. It is operated in CV mode by feeding back the outputvoltage and adjusting the output current to effect the desired change inoutput voltage. CV output, as used herein, is an output with acontrolled voltage that may be used for CV welding. CV power source, asused herein, includes a power source that provides a desired outputvoltage, either constant, or in accordance with a varying waveform.

Alternative embodiments provide that system for providing welding power100 be a CMT or AC MIG power supply and provide a CMT or AC MIG output.CMT output, as used herein, is an output that may be used for CMTwelding. AC MIG output, as used herein, is an output that may be usedfor AC MIG welding. CMT power source, as used herein, includes a powersource that provides a CMT output. AC MIG power source, as used herein,is a power source that provides an AC MIG output. Other alternativesprovide that the sampled output parameter is current, power, loadresistance, or arc length.

Controller 104 controls power supply 102 to provide a cyclical CVoutput, having at least a peak segment and a background segment.Referring now to FIG. 2, an example of a cyclical CV output from asystem for providing welding power in accordance with the presentinvention is shown, where graph 202 is the voltage waveform and graph208 is the current waveform. Segments 204 are the background segmentsand segments 206 are the peak segments. Cyclical welding output, as usedherein, includes welding output that may be characterized a as a seriesof cycles, wherein each cycle may be the same, similar or different.Segment, as used herein, includes a portion of a welding cycle.

Controller 104 is a CV controller and samples the output voltage manytimes (about every 100 μsec or 10,000 times per cycle in the preferredembodiment) each cycle. The sampling occurs during both the backgroundand peak segments. Each cycle has more than 2 segments in variousembodiments, and the sampling occurs in all or less than all of thesegments. Sampling, as used herein, includes feeding back an analogsignal, or a discrete signal, that is responsive to an output parametersuch as voltage, current, arc length, power, or load resistance.

Each output voltage feedback sample is compared to desired outputvoltage in the preferred embodiment. (Multiple samples within one cyclemay be averaged and compared in other embodiments). The difference, orerror, is used in a control loop, such as a PI or PID control loop. Thegain of the control loop is different for each segment, in the preferredembodiment. Thus, the gain for a sample is dependent upon the segment inwhich the sample is taken. Alternative embodiments provide for the samegain, or for different gains within the segment.

The output of the control loop is used to control the output current ofpower source 102 in the preferred embodiment. Changing the outputcurrent effects a change in output voltage, and thus a change in arclength.

FIG. 2 shows a rounding of the corners of the transition betweensegments, which provides a quieter arc. The rounded corners are providedby using a CC control (or partial CC control) scheme near thetransitions to obtain a CC output, and the CV control scheme in a window(a sub-segment of a segment) away from the transitions. A CC control, aCV control, or a combination thereof is used to ramp between peak andbackground in various embodiments.

Another benefit of rounding the corners is a greater immunity tosecondary loop inductance. A large sudden change to current is difficultto achieve in an inductive system, although in a CC ramp mode there maybe sudden changes to the rate of change of current (DI/DT) at the startand stop of each ramp. By maintaining a voltage, and rounding thecorners there are less drastic changes to the current.

CV control, as used herein, includes controlling a power source toprovide a desired output voltage, either constant, or in accordance witha varying waveform. CC control, as used herein, includes controlling apower source to provide a desired output current, either constant, or inaccordance with a varying waveform. CC output, as used herein, is anoutput with a controlled current that may be used for CC welding.

The control loop changes the output in response to a given sample withinthe segment the sample was taken. This is called controlling the processwithin a given segment.

The wave form of FIG. 2 is divided into two segments, each with CVcontrol within a window and CC control outside of the window. The twosegments are peak and background. The waveform could be divided intoadditional segments, each with a gain. The window does not extend to thebeginning or to the end of the segment, and in the preferred embodimentthe window covers 90-95% of the segment, with equal portions outside thewindow on either end. Not extending to the beginning or the end of eachsegment, as used herein, includes not being with 0.5% of the beginningor the end of the segment. In one embodiment the window extends to oneof the beginning and the end of the segment, but not the other.

Controller 104 is taught the desired voltages (from which the errors arederived in conjunction with the sampled voltage) for various wirefeedspeeds and the values are interpolated between the taught points in thepreferred embodiment. In other words the user sets the desired voltagesfor a particular waveform, by downloading a waveform, entering ateaching mode, data entry, etc. The desired data points are preset inother embodiments. Each desired voltage is called a data point.

Controller 104 also commands power source 102 to provide a minimumoutput current, regardless of the feedback loop, so that the arc doesnot extinguish, or go below a spray threshold. When the minimum isreached, other parameters are controlled to move the current away fromthe minimum, so that the CV loop can be used to control the process, inone embodiment.

Referring now to FIG. 3, a block diagram of controller 104 is shown, andincludes a feedback or sampling module 302, a desired voltage teachingmodule (or a fixed data set) 304, an error module 306, a gain module308, a PI module 310 (or a PID module in an alternative embodiment), aminimum current module 312, a CC control module 314, a window module 316and a current command module 318, which cooperate to provide a currentcommand signal to power source 102. Module, as used herein, includessoftware and/or hardware that cooperates to perform one or more tasks,and can include digital commands, control circuitry, power circuitry,networking hardware, etc.

Controller 104 receives a feedback signal on line 303, and provides itto sampling module 302. The feedback signal is indicative of outputvoltage in the preferred embodiment, but may be other parameters asdiscussed above. Output voltage (or other output parameters) may bemeasured at the output studs, in the power supply or wire feeder, at thearc, or other places that indicate the load voltage. The fedback signalmay also be a function of the fedback parameter, such as derivative,integral, power, log, product, difference, etc., or combinationsthereof. Sampling module 302 samples the fedback signal, or it mayreceive discrete fedback samples. Controller 104 is a digital controllerand implemented with software in the preferred embodiment, and samplingmodule 303 includes an A-D converter. It is implemented with hardware(digital or analog) in other embodiments.

The signal indicative of the sampled output parameter is provided fromsampling module 302 to error module 306. The desired voltage from setpoint module or desired voltage teaching module 304 is also provided toerror module 306, which finds the difference therebetween. Set pointmodule 304 is a look up table in the preferred embodiment, and errormodule 306 is digitally implemented in the preferred embodiment. Adesired voltage is set (taught) for a specific process, wire type, size,gas and wire feed rate in the preferred embodiment. Module 304interpolates between taught points. If the desired voltage is left atzero, the CV adaptive loop is not active for that waveform segment.Thus, the CV control can be turned off for a particular segment, or partof a segment.

The “rounding” at the transitions (shown in FIG. 2) can be implementedby setting the “taught” value to zero near the transitions, thus turningoff the CV control near the transitions. A fast CC ramp can be used totransition to a point at which a slower CV rate of ramping (based onvoltage error) can be used to soften corners. This generates a quieterarc (for user appeal) and also makes the system less susceptible to weldcable inductance issues.

The error signal from error module 306 is provided to gain module 308,which applies a gain based on the segment the sample was taken from, inthe preferred embodiment. The gain for the segment is chosen to providea desired response to achieve an output voltage desired for that segment(which also determines the arc length).

The output of gain module 308 is provided to PI control module 310,which applies a PI control scheme and provides as an output a controlsignal (based on a value proportional to the error and the integral ofthe error). Alternative embodiments include a PI control scheme, orcontrol schemes based on other functions of the error.

The output of control module 310 is provided to current command module318 where it is summed with the current command, to effect a change inthe output current, in the preferred embodiment. Current command module318 provides an output current command signal that dictates themagnitude of the current provided by power source 102. The outputcommand dictates output voltage or output power in alternativeembodiments,

Minimum current module 312 provides a minimum current signal to currentcommand module 318, which insures the current is not commanded to bebelow a desired threshold. The minimum current prevents the adaptivescheme from forcing the current down to a point where the arc cannot bemaintained (seen visually in the arc as an arc outage), or to a levelwhere the desired transfer characteristics are lost.

CC control module 314 also provides a signal to current control module318, which provides the CC command, used for CC control. CC controlmodule may also receive feedback, and control in accordance with knowncontrol schemes. The CC command sets the rate of change between segmentsin the preferred embodiment.

Window module 316 provides a window signal to current command module318, indicating if the output wave form is in the window in which the CVcontrol will be applied. The output (state) of window module 316 may beon (inside the window) and current command module 318 sums the CVcommand with the current command, the state may be off (outside thewindow) and current command module 318 does not sum the CV command withthe current command, or it may call for a scaled back CV command to beadded to the current command.

The combined CV and CC controls of the preferred embodiment providesynergistic benefits. The CV ramp prevents the overshoots andundershoots of a strictly CC ramping system. The CC ramp allows thecurrent to change rapidly so that the CV gains can be optimized for theplateau regions of the waveform.

The control modules may be implemented with the following pseudo-code:

 Sample the Actual_Voltage.  If not in Constant Current mode and aConstant  Voltage command exists for this phase   {   Voltage_Error =Desired_Voltage − Actual_Voltage     CV_Adjustment = Voltage_Error *Phase_Specific_Gain     New_Welder_Output_Command =Old_Welder_Output_Command + CV_Adjustment  if (New_Welder_Output_Command< Minimum_Current)      {        New_Welder_Output_Command =Minimum_Current ;      }  }

One aspect of the present invention is to use the CV control andcontroller 104 as an arc length controller. Arc length controller, asused herein, is a controller that controls the output of a system forwelding such that the arc length is controlled to be a desired length,preferably a constant length. This is done regardless of changes totorch angle, and stickout. FIG. 4 shows stickout, arc angle, and arclength.

The arc length controller may be implemented with the CV control becausearc length generally follows arc voltage. It may also be implementedusing other welding output parameters, such as output current, outputvoltage, output power, load resistance, etc., as the fedback parameter.

The frequent sampling and changing the output in the same cycle providesa very fast response to changes in arc length. This provides a tightermore controllable weld, without the undesirable spatter. The tighter arcallows good weld penetration into the weld piece, but does not have abexcessively fluid weld puddle. The arc length controller embodimenthelps the welder weld in a variety of orientations, including verticallyup or down.

A minimum peak current setting is provided in one alternative. Thiswould prevent the CV control from driving the peak current so low thatthe ball (of molten metal) is not detached.

Other alternatives provide that the user can change the frequency (by afine adjustment knob) to add or subtract arc length for user fine-tuningof the arc. In another alternative, it may be possible to adjust thepeak and background voltages, and/or adjust relative times of high andlow voltage segments as a different method of adjusting the arc length.

Yet another alternative applies the control scheme to melt rate or amathematical representation of arc length.

Numerous modifications may be made to the present invention which stillfall within the intended scope hereof. Thus, it should be apparent thatthere has been provided in accordance with the present invention amethod and apparatus for arc welding that fully satisfies the objectivesand advantages set forth above. Although the invention has beendescribed in conjunction with specific embodiments thereof, it isevident that many alternatives, modifications and variations will beapparent to those skilled in the art. Accordingly, it is intended toembrace all such alternatives, modifications and variations that fallwithin the spirit and broad scope of the appended claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A welding power supply,comprising: a CV power source, having a cyclical CV output with avoltage; and means for controlling the CV power source, including meansfor dividing each cycle into at least two segments, means for samplingan output parameter a plurality of times within at least a one of thesegments, and means for controlling the CV output within the at leastone segment in response to the sampling performed within the at leastone segment, wherein the means for sampling includes means for samplingthe parameter a plurality of times within at least a second of thesegments, and the means for controlling the CV output includes means forcontrolling the CV output within the at least a second segment inresponse to the sampling performed within the at least a second segment.2. The welding power supply of claim 1 wherein the means for samplingincludes means for sampling the parameter a plurality of times withineach of the segments, and the means for controlling the CV outputincludes means for controlling the CV output within each of the segmentsin response to the sampling performed within each segment.
 3. Thewelding power supply of claim 2, further comprising means for comparingthe sampled voltage to a desired voltage and obtaining an errortherefrom, and wherein the means for controlling responsive to theerror.
 4. The welding power supply of claim 3, further comprising meansfor applying a gain to the error, wherein the gain is dependent upon thesegment in which the sampling is taken.
 5. The welding power supply ofclaim 4, further comprising means for setting the desired voltage byteaching at a plurality of data points.
 6. The welding power supply ofclaim 2, wherein the means for controlling includes means for respondingonly to sampling that is not from at least one of the beginning and theend of the at least one segment.
 7. The welding power supply of claim 6,further comprising means for providing a CC output at the beginning andend of the at least one segment.